We present a measurement of angular observables and a test of lepton flavor universality in the $B\to K^{*}{\ell }^{+}{\ell }^{-}$ decay, where $\ell$ is either $e$ or $\mu$. The analysis is performed on a data sample corresponding to an integrated luminosity of $711{\mathrm{fb}}^{-1}$ containing $772\times 10^6$ $B\overline{B}$ pairs, collected at the $\mathrm{\Upsilon }(4S)$ resonance with the Belle detector at the asymmetric-energy $e^{+}{e}^{-}$ collider KEKB. The result is consistent with standard model (SM) expectations, where the largest discrepancy from a SM prediction is observed in the muon modes with a local significance of $2.6\sigma$.

We report a measurement of the ratio $\mathcal{R}(D^{*})=\mathcal{B}({\overline{B}}^0\to D^{*+}{\tau }^{-}{\overline{\nu }}_{\tau })/\mathcal{B}({\overline{B}}^0\to D^{*+}{\ell }^{-}{\overline{\nu }}_{\ell })$, where $\ell$ denotes an electron or a muon. The results are based on a data sample containing $772\times 10^{6}B\overline{B}$ pairs recorded at the $\mathrm{\Upsilon }(4S)$ resonance with the Belle detector at the KEKB $e^{+}{e}^{-}$ collider. We select a sample of $B^0{\overline{B}}^0$ pairs by reconstructing both $B$ mesons in semileptonic decays to $D^{*\mp }{\ell }^{\pm }$. We measure $\mathcal{R}(D^{*})=0.302\pm 0.030(\text{stat})\pm 0.011(\text{syst})$, which is within $1.6\sigma$ of the Standard Model theoretical expectation, where the standard deviation $\sigma$ includes systematic uncertainties. We use this measurement to constrain several scenarios of new physics in a model-independent approach.

With the full data sample of $772\times {10}^{6}B\overline{B}$ pairs recorded by the Belle detector at the KEKB electron-positron collider, the decay $\overline{B}\to D^{*}{\tau }^{-}{\overline{\nu }}_{\tau }$ is studied with the hadronic $\tau$ decays ${\tau }^{-}\to {\pi }^{-}{\nu }_{\tau }$ and ${\tau }^{-}\to {\rho }^{-}{\nu }_{\tau }$. The $\tau$ polarization $P_{\tau }(D^{*})$ in two-body hadronic $\tau$ decays is measured, as well as the ratio of the branching fractions $R(D^{*})=\mathcal{B}(\overline{B}\to D^{*}{\tau }^{-}{\overline{\nu }}_{\tau })/\mathcal{B}(\overline{B}\to D^{*}{\ell }^{-}{\overline{\nu }}_{\ell })$, where ${\ell }^{-}$ denotes an electron or a muon. Our results, $P_{\tau }(D^{*})=-0.38\pm 0.51{(\text{stat})}_{-0.16}^{+0.21}(\text{syst})$ and $R(D^{*})=0.270\pm 0.035{(\text{stat})}_{-0.025}^{+0.028}(\text{syst})$, are consistent with the theoretical predictions of the standard model. The polarization values of $P_{\tau }(D^{*})>+0.5$ are excluded at the 90% confidence level.

We present the results of a search for the rare decays $B\to h\nu \overline{\nu }$, where $h$ stands for $K^{+},K_{\mathrm{S}}^0,{K^{*}}^{+},{K^{*}}^0,{\pi }^{+},{\pi }^0,{\rho }^{+}$ and ${\rho }^0$. The results are obtained with $772\times {10}^{6}B\overline{B}$ pairs collected with the Belle detector at the KEKB $e^{+}{e}^{-}$ collider. We reconstruct one $B$ meson in a semileptonic decay and require a single $h$ meson but nothing else on the signal side. We observe no significant signal and set upper limits on the branching fractions. The limits set on the $B^0\to K_{\mathrm{S}}^0\nu \overline{\nu }$, $B^0\to {K^{*}}^0\nu \overline{\nu }$, $B^{+}\to {\pi }^{+}\nu \overline{\nu }$, $B^0\to {\pi }^0\nu \overline{\nu }$, $B^{+}\to {\rho }^{+}\nu \overline{\nu }$, and $B^0\to {\rho }^0\nu \overline{\nu }$ channels are the world’s most stringent.

The experimental results on the ratios of branching fractions $\mathcal{R}(D)=\mathcal{B}(\overline{B}\to D{\tau }^{-}{\overline{\nu }}_{\tau })/\mathcal{B}(\overline{B}\to D{\ell }^{-}{\overline{\nu }}_{\ell })$ and $\mathcal{R}(D^{*})=\mathcal{B}(\overline{B}\to D^{*}{\tau }^{-}{\overline{\nu }}_{\tau })/\mathcal{B}(\overline{B}\to D^{*}{\ell }^{-}{\overline{\nu }}_{\ell })$, where $\ell$ denotes an electron or a muon, show a long-standing discrepancy with the standard model predictions, and might hint at a violation of lepton flavor universality. We report a new simultaneous measurement of $\mathcal{R}(D)$ and $\mathcal{R}(D^{*})$, based on a data sample containing $772\times {10}^{6}B\overline{B}$ events recorded at the $\mathrm{\Upsilon }(4S)$ resonance with the Belle detector at the KEKB $e^{+}{e}^{-}$ collider. In this analysis the tag-side $B$ meson is reconstructed in a semileptonic decay mode and the signal-side $\tau$ is reconstructed in a purely leptonic decay. The measured values are $\mathcal{R}(D)=0.307\pm 0.037\pm 0.016$ and $\mathcal{R}(D^{*})=0.283\pm 0.018\pm 0.014$, where the first uncertainties are statistical and the second are systematic. These results are in agreement with the standard model predictions within 0.2, 1.1, and 0.8 standard deviations for $\mathcal{R}(D)$, $\mathcal{R}(D^{*})$, and their combination, respectively. This work constitutes the most precise measurements of $\mathcal{R}(D)$ and $\mathcal{R}(D^{*})$ performed to date as well as the first result for $\mathcal{R}(D)$ based on a semileptonic tagging method.